Just outside of Los Altos is a very long ridge of mountains, one of which is Black Mountain. 

This is a very dry hike, with no tree canopy, so it can get very hot! Be aware that this can make the hike feel a lot more difficult with the sun beating down on you. Follow Rhus Ridge Trail to Black Mountain Trail, then continue on Black Mountain Trail to reach the summit. One way to the summit is almost 5 miles, so make sure to make enough time for this hike! 

The trail has a very gradual incline, up until the very last part -- don't think you're done until you're actually done! Not too many views exist at the very top, where you'll find some radio towers, but on the way down, you'll see some expansive views of Silicon Valley.